Background

Animals in early stages of development are particularly susceptible to harmful effects caused by toxic pollutants. For this reason, the transfer of toxic pollutants from mothers to their young, a process known as maternal offloading, has been the subject of intense research. However, little is known about how non-mammalian species like sharks, skates, and rays pass toxic pollutants on to their young. Researchers from California State University in Long Beach investigated maternal offloading in round stingrays by comparing contaminant burdens in mothers and their developing offspring.

Many toxic pollutants can bioaccumulate. These contaminants are not easily degraded, so they remain in the body until they are mobilized, which most often happens when those lipids are used by the organism. This often occurs when animals undergo energetically costly tasks. For example, when animals undergo intense migrations and cease eating they burn up fat stores, potentially releasing accumulated toxins. This remobilization can also occur when pregnant females produce nourishing materials and transfer them to their young. The materials passed on by the mother, such as milk or egg yolk, are very fatty and lipid-rich substances. When they are mobilized, toxic contaminants come along for the ride.

To date, most of the research on maternal offloading has focused on mammals, including whales, seals, and humans. For mammals, the bulk of maternal offloading actually occurs during nursing, when the offspring consume lipid-rich milk. The transfer of contaminants between mother and offspring for other types of organisms that don’t nurse is generally less well characterized.

Round stingrays are elasmobranchs, a subclass of cartilaginous fish which includes sharks and skates. Their young are nourished in two different ways during development: during early stages they consume an egg yolk, and in later stages nutrients are transferred via secreted fluids derived from the mother’s liver. These fluids are known as histotroph, or “uterine milk”. This study aimed to determine whether contaminants were transferred to young during both of these developmental stages and how contaminant levels in developing offspring compared to levels in mothers.

Methods

Researchers collected round stingrays off the coast of California. To determine whether nourishment pathways pass toxics on to developing stingrays, they analyzed mothers and young at different stages of development for the pesticide DDT, as well as its metabolites, and for polychlorinated biphenyls (PCBs). These compounds were banned in the 1970s but are still commonly found in marine organisms all over the world due to their environmental persistence.

Results

Researchers found that the overwhelming majority of contaminants present in the adult round stingrays were accumulated in the liver, so they used the pollutant concentration measured in the liver to represent total body burden. The initial amount of contamination measured in the mother was an important factor in determining the concentrations of toxic compounds in the young. This means that mothers who have been exposed to higher levels of contaminants, perhaps due to feeding in a highly impacted area, are likely to pass on higher concentrations to their offspring.

The researchers also found that a higher percentage of the mother’s total DDT burden was transferred to offspring, compared to PCBs. This suggests that PCBs were offloaded less efficiently than DDT. This could occur because the DDT molecule has a higher affinity for transport into lipids used to make egg yolk or uterine milk than PCB compounds do.

Figure 1: Researchers found that levels of contamination increased throughout the development of stingray offspring. Here, OC load/# refers to the concentration of organic contamination measured per individual at different developmental stages.

Figure 1 shows a significant increase in contamination as offspring develop from ova, to eggs, to embryos. Contaminant levels continued increasing past the point when the young would have finished consuming the egg yolk (about 1 month into gestation). These results suggest that stingrays transfer pollutants to their young through early and late stages of gestation. First, offspring receive pollutants during consumption of the egg yolk. Later, they continue to receive pollutants from uterine milk.

Researchers also found that mothers in the early stages of pregnancy had significantly higher concentrations of PCBs and DDT than mothers in the late stages of pregnancy, suggesting once again that they had passed some of their contaminant burden to their young as development progressed. However, the percentage of total contaminants offloaded was only about 1.5 +/- 1.7% of the total amount accumulated in mothers.

Significance

For scientists to understand the impacts of manmade toxic chemicals on sea creatures, they must take maternal offloading into account. Developing offspring are especially vulnerable to environmental toxins. Exposure could result in hormonal changes or developmental deficiencies that could have very serious consequences for the future of a species.

This study sheds some light on how maternal offloading could affect non-mammalian species like the elasmobranchs. Results from this study can be used to understand effects of environmental toxins on other members of this diverse and fascinating group, many of which are rare and difficult to study. Many of these creatures are already struggling due to habitat loss and prey scarcity. This study suggests maternal offloading of toxic contaminants could also be important in determining the future health of these li’l critters.

I am the founder of oceanbites, and a postdoctoral fellow in the Higgins Lab at Colorado School of Mines, where I study poly- and perfluorinated chemicals. I got my Ph.D. in the Lohmann Lab at the University of Rhode Island Graduate School of Oceanography, where my research focused on how toxic chemicals like flame retardants end up in our lakes and oceans. Before graduate school, I earned a B.Sc. in chemistry from MIT and spent two years in environmental consulting. When I’m not doing chemistry in the lab, I’m doing chemistry at home (brewing beer).